CNG Car Conversions: Will I Pass My State Safety, Emissions and Smog Test?

One of the most common questions people have revolves around state safety and emissions testing. In some states called the “smog” or “tailpipe” test. The SkyCNG systems will not tamper with or disable your vehicles “smog” or oxygen sensors… nor do they modify your vehicle PCM or block the check engine light. SkyCNG kits constantly communicate with your vehicle’s OBD to run at optimal levels. When done right, you won’t get a check engine light or run into trouble at your annual safety/emissions test.

Most people know that CNG is a cleaner burning fuel than gasoline. CNG at the pump is over 90% methane gas. It burns up and leaves very little behind. Gasoline burns up and leaves quite a bit behind, including unburned gasoline. This is why automakers in the USA have been installing the CATALYTIC CONVERTER on car exhaust since the 1970s. The catalytic converter literally converts a lot of the leftover stuff from gasoline into carbon dioxide (CO2) and water (H2O).

Critics and pretty much most smart people will tell you that those catalytic converters suck the life and performance out of your engine by reducing horsepower and quite literally choking up the gasoline combustion. This is a topic for another article, so let’s stick with CNG vs. gasoline in your emissions…

Will my car pass emissions testing after a CNG conversion kit is installed? We can show you the answer is yes, assuming it runs well on gasoline, too.


What Happens If I Get An Emissions and Safety Test?

SkyCNG went to our local Jiffy Lube and got an emissions test performed. You can see the full results for yourself.This 2010 Toyota Tundra 5.7 liter V8 has had a SkyCNG conversion on it for about 12,000 miles. There are 64k miles on the engine. Everything passed with flying colors.

You’ll notice the CO2 emissions were reduced by over 20%. However, you’ll also notice the “HC” or the hydrocarbons were actually INCREASED while on CNG. Let’s keep in mind that everything was still well below the acceptable levels, but why the increase in hydrocarbons?

We don’t want to get too technical, but the short answer is that a vehicle’s catalytic converter is specifically “tuned” to pull out and “convert” certain particles only found in gasoline exhaust. When you start running the clean burning CNG through the exhaust pipes your catalytic converter doesn’t do much. It still chokes up the engine, but the chemical reactions aren’t taking place like with gasoline. Some of the “unburned” methane (methane is a hydrocarbon) could be showing up on your emissions test and making it look “dirtier”. Just remember, the levels are still very low and PASSING GRADE and the CO2 levels are much lower.

A driver can also reduce these hydrocarbon levels by getting the installer to perform a really good tuning and calibration, as well as a high quality CNG conversion system. If the engine is running a little bit rich, then there will be more “unburned” CNG and therefore higher HC levels.

Dedicated natural gas vehicles (NGV) will show almost no emissions since they are equipped with a methane-specific type of catalytic converter. It is not practical to have both types of converters on a bi-fuel conversion engine.

Let us know if you have any questions!

What Happens If I Get An Emissions and Safety Test?

Many states don’t even have these emissions tests, but do not fear. If your kit was installed properly you will not have an issue passing the OBD test and/or tailpipe test. No matter where you live in the US, though, you WILL need a reliable CNG conversion kit and cylinder tank install. Additionally, you’ll need a blue CNG diamond sticker visable to pass NFPA 52 regulations (learn more about¬†NFPA 52 regulations here). This sticker gives emergency crews a heads up in the event of an accident involving your vehicle. It would also be a good for a mechanic to know, so make sure the sticker is put on the rear bumper to comply with NFPA 52. EPA certified or not, you NEED this sticker on your NGV.

Another minimal “test” you’ll need no matter where you live is a CNG cylinder inspection. Get this done every 3 years or 36,000 miles by a CSA Certified Inspector. Your local installer will likely have this certification.

Finally, never tamper with your car’s sensors or catalytic converter. DO IT RIGHT!

Other states have annual or bi-annual safety/smog tests. We have learned that states like Arizona (according to a few of our readers) only test your vehicle on the fuel its running when dropped at the shop. Bring your NGV in running on gasoline (assuming it is registered as a gasoline vehicle). The mechanic will hook up to your tailpipe and the properly installed CNG conversion kit will not effect your smog test. Remember to research your local state laws before you try to qualify for the HOV lane, special license plates, or other clean fuel perks. If you do not have an EPA certified conversion kit then you likely will not qualify (but who cares if you saved thousands by getting a non-EPA certified kit). If you try to get the perks, you’ll be asked for proof of certification which you won’t have.

The Utah Highway Patrol oversees the safety inspection in their state and the CNG tank/system must be visually inspected every 3 years or 36,000 miles (whichever comes first) by a certified CNG inspector. The inspector does not “report” to anybody. The inspector gives you a written report of the inspection. If you fail the inspection, the inspector does not take your keys, but the written report says you failed the test. When presented to the mechanic performing the state safety inspection you will not be able to register your vehicle until the issue has been resolved. Nobody tells the EPA. Some drivers have told me they pulled in running on gasoline, got the test, and pulled out running on gasoline holding their new license plate sticker and registration.

If you have tampered with your sensors though, you will likey fail no matter what fuel you are running on. Aspirated or “mixer” style kits (the older generation of CNG conversions) usually have more issues passing inspections.

Californians… thanks to another layer of goverment called CARB (California Air Resources Board) you will have to spend more money than anyone getting your state inspections. When you bring your bi-fuel NGV in for testing the mechanic will see you have a CNG system installed and he will ask for your CARB approvals. You will NOT pass inspections without these approvals. The vehicle will have to go to a referee for the inspection and get a “Referee label”. You will not get these approvals from CARB referees/inspectors unless you have an EPA certified CNG conversion kit installed on a qualifying vehicle. Remember after all this, you still have to pass smog tests on gasoline and CNG.